1. Field of the Invention
The present invention relates to a dielectric filter, more particularly to a BEF type dielectric filter which is applicable, but not exclusively, to mobile communication apparatus and the like.
2. Description of the Prior Art
FIG. 7 illustrates a structure of a conventional one-stage band-elimination filter (which will be referred hereinafter to as a one-stage BEF), which includes two inter-digitally-coupled resonant lines. In FIG. 7, to form the one-stage BEF, .lambda./4 resonant lines 11a, 11b are inter-digitally coupled such that their open ends and short-circuit ends are arranged to be opposite in direction to each other in a dielectric block 20. Further, FIG. 8 shows its circuit arrangement, FIG. 9 illustrates an equivalent circuit, and FIG. 10 is a cross-sectional view, taken along line X--X of FIG. 7, showing how the equivalent circuit is formed.
In FIG. 10, unit length self-capacitances C11 are formed between the resonant line 11a and an external conductor and between the resonant line 11b and the external conductor, respectively. In addition, an inter-line mutual capacitance C12 is defined between the resonant lines 11a and 11b. In FIGS. 8 to 10, the references are as follows:
Zin=input impedance PA1 Ze (characteristic impedance in even mode)=.sqroot.(.epsilon.)/(vc.times.C.sub.11) PA1 where er: dielectric constant
vc: velocity of light PA2 C.sub.11 : unit length self-capacitance (see FIG. 10) ##EQU1## where Zo (characteristic impedance in odd mode) ##EQU2##
In the FIG. 9 equivalent circuit, between the input (or output) and ground, the even mode characteristic impedance Ze is connected in parallel to the series circuit made up of the coupling characteristic impedance Zk and even mode characteristic impedance Ze.
Furthermore, FIG. 11 shows an input impedance characteristic of the one-stage BEF, and FIG. 12 illustrates an attenuation characteristic. In FIG. 12, the trap frequency fT is expressed in accordance with the following equation. EQU f.sub.T =vc/4.sqroot.(.epsilon.).times.L
As seen in FIGS. 11 and 12, the impedance becomes zero at the trap frequency fT. This is because the reflective phase creates a short.
However, it is not possible to make an antenna coupler using a one-stage BEF having such a characteristic in a transmission or reception side. That is, as seen in FIG. 11, when viewed from the transmission side filter, for example, the impedance of the reception side filter becomes zero at the passband of the transmission side filter, so the signal does not flow to the antenna. The same problem may exist when the impedance of the transmission side filter becomes zero at the passband of the reception side filter.
For an antenna coupler by use of the one-stage BEF, as illustrated in FIG. 13, a phase-shifting line has conventionally been needed on either the transmission side TX or reception side RX. This increases the number of parts and, thereby, raises the cost of the antenna coupler.